A transmission electron microscopy analysis of secondary minerals formed in tungsten-mine tailings with an emphasis on arsenopyrite oxidation |
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| Institution: | 1. Facultad de Ciencias Químicas, Departamento de Ciencia de Materiales, Universidad Juárez del Estado de Durango (UJED), Av. Veterinaria S/N, Circuito Universitario, Col. Valle del Sur, 34120 Durango, Dgo, Mexico;2. Instituto Tecnológico de Durando, UPIDET, Av. Felipe Pescador 1830 Ote. Col. Nueva Vizcaya, 34080 Durango, Dgo, Mexico;3. Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco 186, Col. Vicentina, Iztapalapa, México DF 09340, Mexico;4. Instituto Politécnico Nacional, UPALM, Edif. Z-4 3er Piso, CP 07738 México D.F, Mexico;5. Departamento de Biotecnología, Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco 186, Col. Vicentina, Iztapalapa, México DF 09340, Mexico;6. Universidad Autónoma Metropolitana-Azcapotzalco, Área de Crecimiento Económico y Medio Ambiente, Departamento de Economía, Av. San Pablo 180, Azcapotzalco, México DF 02200, Mexico;7. CIACyT, Universidad Autónoma de San Luis Potosí, Av. Sierra Leona 550, Lomas 2da sección, 78230 San Luis Potosí, SLP, Mexico |
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| Abstract: | Grains of naturally oxidized arsenopyrite FeAsS] collected from the oxidation zone in W-mine tailings were investigated, primarily using transmission electron microscopy. The grains are severely pitted and are surrounded by secondary minerals. The pitted nature of the grains is related to mechanisms governing the electrochemical oxidation of sulfide minerals, with prominent cusp-like features occurring at cathodic regions of the surface, and pits occurring at anodic regions. In general, the oxidation of arsenopyrite leads to the formation of an amorphous (or nanocrystalline) Fe–As–O-rich coating that contains small amounts of Si, Ca, Cu, Zn, Pb and Bi; nanoscale variation in the As, Pb, Bi and Zn contents of the coating was noted. Secondary Cu sulfides, thought to be chalcocite Cu2S] and (or) djurleite Cu31S16], occur as a layer (generally <500 nm thick) along the arsenopyrite grain boundary, and also within the coating as aggregates, and as layers that parallel the grain boundary. Although the precipitation of secondary Cu minerals along the grain boundary is a nanoscale feature, the process of formation is thought to be analogous to the supergene enrichment that occurs in weathered sulfide deposits. As the oxidation of arsenopyrite proceeds, layers and clusters of secondary Cu sulfides become isolated in the Fe–As–O coating. Secondary wulfenite PbMoO4] and an unidentified crystalline Bi–Pb–As–O mineral occur in voids within the coating, suggesting that these minerals precipitated from the local pore-water. Small and variable amounts of W, Ca, Bi, As and Zn are associated with the wulfenite, and Zn, Fe and Ca are associated with the Bi–Pb–As–O mineral. Some of the wulfenite is in contact with inclusions of molybdenite MoS2], suggesting that the oxidation of molybdenite in the presence of aqueous Pb(II) led to the formation of wulfenite. Mineralogical analyses at the nanoscale have improved the understanding of geochemical sources and sinks at this location. The results of this study indicate that the mineralogical controls on aqueous elemental concentrations at this tailings site are complex and are not predicted by thermodynamic calculations. |
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